An international research team led by the University of Electronic Science and Technology of China (UESTC) has used pyrrodiazole (PZ) additives in formamidinium iodide (FAI)-based inverted perovskite solar cells increasing perovskite film stability, enabling slot die coating in air.
Inverted perovskite cells have a device structure known as “p-i-n”, in which hole-selective contact p is at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the HTL surface.
“We achieved the scalable preparation of homogeneous structure perovskite films via regulating the crystallization process of perovskite through immobilizing strategy,” corresponding author, Shibin Li, told pv magazine.
Both lead iodide (PbI2) and formamidinium iodide (FAI) benefitted from the PZ immobilization strategy, enabling the formation of “high-quality” perovskite films for the cells. “The immobilization effect, driven by the formation of Lewis acid-base pairs (PZ-PbI2) and hydrogen bonds (PZ-FAI), not only improves the colloidal size distribution uniformity in the perovskite precursor solution but also enhances the stability of the wet film by inhibiting colloid agglomeration,” the group explained.
The cell stack was as follows: transparent fluorine-doped tin oxide (FTO) coated glass substrate, a sputtered nickel(II) oxide (NiOx) film, the methyl-substituted carbazole (Me-4PACz) layer, perovskite, buckminsterfullerene (C60) electron transport layer (ETL), a bathocuproine (BCP) buffer layer, copper (Cu) contact.
The Me-4PACZ film was coated on the NiOx film surface with blade coating in air. “After that, the prepared perovskite precursor solution with different concentrations of additives were deposited in air by slot-die coating technology,” said the team.
Real-time characterization revealed that “this approach not only retards the crystal growth process but also ensures consistent crystallization rates between the upper and lower layers of the perovskite film,” the researchers explained. The process facilitates formation of large, monolithic grains with “excellent uniformity.”
Based on this approach, the resulting mini modules achieved a maximum efficiency of 21.5% and a certified efficiency of 20.3%, which ranks among the highest certified efficiencies reported for mini inverted perovskite modules with an aperture area of over 50 cm2.
The results were confirmed by China's National Institute of Measurement and Testing Technology. The panels were also found to retain 94% of initial efficiency after continuous light aging in air for 1000 h under a relative humidity of 65%. “Our approach enhances wet film stability by suppressing colloidal aggregation, retarding crystal growth, which ensures a consistent growth rate across the films,” the scientists said.
The inverted perovskite solar mini modules in the experiment measured 10 cm × 10 cm. Each had 11 cells connected in series. It is worth noting, stated the researchers, that the P1, P2, and P3 scribing of the series-connected modules, had P1 and P3 dividing the module sub-cells, with P2 connecting the sub-cells.
Further tests revealed that, in modules with an aperture area of 56.5 cm2, PZ improved module efficiency from 18.2% to 21.5%, while PZ-I and PZ-II either had negligible effects (17.9%) or reduced efficiency (17.0%).
The details of the research appear in “Scalable preparation of perovskite films with homogeneous structure via immobilizing strategy for high-performance solar modules,” recently published in nature communication. The researchers were from the University of Electronic Science and Technology of China (UESTC), Guangzhou University, China Jiliang University, and Institut de Recherche de Chimie Paris (IRCP) in France.
“We will focus on optimizing the efficiency and stability of large-area perovskite-silicon tandem solar cells,” Li said, referring to the future direction of the research team.
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